Improve `PreImages` error message if object is not in range

[fingolfin]

Consider this example (originally due to @laurentbartholdi, see also http://tracker.gap-system.org/issues/391):

gap> g := SymmetricGroup(3);;
gap> f := GroupHomomorphismByImages(Group((1,2)),Group((2,3)),[(1,2)],[(2,3)]);
[ (1,2) ] -> [ (2,3) ]
gap> PreImages(f,Group((2,3)));
Group([ (1,2) ])
gap> PreImages(f,Group((2,4)));
Error, usage: PreImages(<map>), PreImages(<map>,<img>), PreImages(<map>,<coll>)

The error message is not very helpful. It would be much more helpful if we said something like "the set/object/list you provided is not contained in the range of the map" or so.

Without thinking too much about it, couldn't we achieve this by modifying e.g. the test

        if     FamRangeEqFamElm( FamilyObj( map ), FamilyObj( img ) )
           and img in Range( map ) then

            return PreImagesElm( map, img );

to this:

        if FamRangeEqFamElm( FamilyObj( map ), FamilyObj( img ) ) then
           if not img in Range( map ) then
             Error("the provided element is not contained in the range of the map"):
           fi;
           return PreImagesElm( map, img );

and so on?

[stertooy]

As of right now, we instead get

gap> PreImages(f,Group((2,4)));
Error, the collection <elm> must be contained in Range(<map>)

Though this behaviour may (or may not) change in the future with PR #6409.